1. Field of the Invention
The present invention relates generally to a method of effecting supplemental bending of an already bent workpiece to achieve a desired overall bending of the workpiece, and an apparatus for determining information required for effecting the supplemental bending, and more particularly to technologies for improving the accuracy of the supplemental bending.
2. Discussion of the Related Art
A product such as a pipe or tube having a centerline extending between the opposite ends is manufactured by bending the appropriate blank or workpiece at a certain position along its centerline between the opposite ends of the workpiece. An example of such a product is each of a plurality of branches of an intake or exhaust manifold attached to an engine of a motor vehicle. This type of product has a bend or bends formed so that an actual relative position (positional relationship) between the opposite ends of the product coincides with a desired or nominal relative position (positional relationship). In the case of a manifold branch, for example, the opposite ends are the inlet and outlet ends.
However, a bending operation on the workpiece so as to achieve the nominal relative position of the opposite ends of the product to be manufactured will not necessarily result in satisfactory coincidence or alignment of the actual relative position of the product with the nominal relative position. There are several causes for failure to achieve the satisfactory coincidence, which include a spring-back phenomenon of the workpiece itself due to its elasticity or resiliency. In the light of this drawback, the assignee of the present invention proposed the following technique as disclosed in JP-A-63-36928 (published in 1988). This technique includes a step of measuring the amount of spring-back of the workpiece (product) upon removal of a bending force from the workpiece at the end of an initial bending operation on the workpiece, and a step of effecting a secondary bending operation on the initially bent workpiece (intermediate product) so as to bend the workpiece by an amount corresponding to the measured spring-back amount. This secondary bending operation may be considered to be a supplemental bending operation to correct the initial bending such that the initial bending error is reflected on the supplemental bending operation so as to improve the overall bending accuracy.
However, the supplemental bending technique indicated above has a problem. That is, the supplemental bending takes place at the same position as the initial bending, and the same portion of the workpiece is subjected to the initial and supplemental bending operations. The two bending operations on the same portion of the workpiece may more or less cause work hardening of the workpiece, which tends to damage the workpiece at the bending position. Where the workpiece takes the form of a pipe, in particular, the pipe tends to also suffer from reduction in the wall thickness and consequent damage at the bending position.
The above supplemental bending technique has another problem. Namely, the technique includes the measurement of the spring-back amount of the initially bent workpiece at the initial bending position, and the supplemental bending operation to achieve the desired overall bending angle or amount with the measured spring-back amount taken into account. However, the error in the actual relative position between the opposite ends of the product increases with an increase in the error in the overall bending angle or amount. Where the relative position of the opposite ends of the product is important, even a small amount of error in the overall bending angle tends to have a significant effect on the actual relative position. In this respect, it is noted that the error in the actual relative position due to the error in the overall bending angle increases as the longitudinal dimension of the workpiece between the opposite ends increases. Therefore, the proposed supplemental bending technique which does not directly rely on the actual relative position of the initially bent workpiece suffers from difficulty to correct or rectify the initial bending with a sufficiently high degree of accuracy so as to achieve the desired or nominal relative position between the opposite ends of the product.